Visitor Pattern with Templated Visitor

Question

This is a follow up on No user defined conversion when using standard variants and visitor pattern

I need to implement a templated version of the visitor pattern as shown below, however it looks like the accept function has to be virtual which is not possible. Could you please help me?

#include <variant>
#include <iostream>

class Visitable //I need this to be non-templated (no template for Visitable!!): Otherwise I could use CRTP to solve this issue.
{
public:
    virtual ~Visitable() = default;

    template<typename Visitor> 
    /*virtual*/ double accept(Visitor* visitor) //I can't do virtual here.
    {
        throw("I don't want to end up here");
    };

protected:
    Visitable() = default;
};

struct DoubleVisitable : public Visitable
{
    template<typename Visitor> 
    double accept(Visitor* visitor) 
    {
        return visitor->visit(*this);
    };

    double m_val = 1.0;
};

struct StringVisitable : public Visitable
{
    template<typename Visitor> 
    double accept(Visitor* visitor) 
    {
        return visitor->visit(*this);
    };
    double m_val = 0.0;
};

template<typename... args>
class Visitor
{
public:
    virtual ~Visitor() = default;

    virtual double visit(typename std::variant<args...> visitable)
    {
        auto op = [this](typename std::variant<args...> visitable) -> double { return this->apply(visitable); };
        return std::visit(std::ref(op), visitable);
    }

    virtual double apply(typename std::variant<args...> visitable) = 0;

    Visitor() = default;
};

class SubVisitor : public Visitor<DoubleVisitable, StringVisitable>
{
public:
    virtual ~SubVisitor() = default;
    SubVisitor() : Visitor<DoubleVisitable, StringVisitable>() {};
    
    virtual double apply(std::variant<DoubleVisitable, StringVisitable> visitable) override
    {
        return std::visit(            
            [this](auto&& v){return process(v);},
            visitable
        );
    };

    virtual double process(const StringVisitable& visitable)
    {
        std::cout << "STRING HANDLED" << std::endl;
        return 0.0;
    }

    virtual double process(const DoubleVisitable& visitable)
    {
        std::cout << "DOUBLE HANDLED" << std::endl;
        return 1.0;
    }
};



int main(int argc, char* argv[])
{
    SubVisitor visitor;
    DoubleVisitable visitable;
    visitable.accept(&visitor);

    //I want to be doing this:
    Visitable* doubleV = new DoubleVisitable();
    doubleV->accept(&visitor);
    delete doubleV;
    return 1;
}

The code is here Link. Could you please help me make this not throw but collapses to the right child class DoubleVisitable or StringVisitable. It looks like I need virtual templated member function which is not possible as mentioned here Can a class member function template be virtual?

Answer 1

In C++, there are no template virtual functions. This does not exist. What you can do is either:

• have an accept method for each class you'd like to visit (each descendant)
• have a std::variant<> of implementations instead of inheritance.

Answer 2

It says in the question that Visitable cannot be a template. But is it allowed to inherit from a template class? And do you know all the possible visitors? If so, you could add a new template class that Visitable inherits from and that declares virtual methods for all the visitors:

template <typename ... T> class AcceptMethods {};
template <> class AcceptMethods<> {};
template <typename First, typename ... Rest>
class AcceptMethods<First, Rest...> : public AcceptMethods<Rest...> {
public:
  virtual double accept(First* ) = 0;
  virtual ~AcceptMethods() {}
};

typedef AcceptMethods<SubVisitor> AllAcceptMethods;

class Visitable : public AllAcceptMethods
{
public:
    virtual ~Visitable() = default;
};

In the above code, we are just listing SubVisitor, but AcceptMethods is variadic so it could be typedef AcceptMethods<A, B, C, D, AndSoOn> AllAcceptMethods;.

Then we add another template class WithGenericAcceptMethod whose purpose is to implement the accept methods declared by AcceptMethods by calling a template method acceptT:

template <typename This, typename ... T> class WithGenericAcceptMethod {};
template <typename This> class WithGenericAcceptMethod<This, AcceptMethods<>> : public Visitable {};
template <typename This, typename First, typename ... Rest>
class WithGenericAcceptMethod<This, AcceptMethods<First, Rest...>> : public WithGenericAcceptMethod<This, AcceptMethods<Rest...>> {
public:
  double accept(First* visitor) override {
    return ((This*)this)->template acceptT<First>(visitor);
  }
  virtual ~WithGenericAcceptMethod() {}
};

This class takes as first argument a This parameter in the spirit of CRTP. Then we can now let the specific visitable classes inherit from WithGenericAcceptMethod and implement the template acceptT method:

struct DoubleVisitable : public WithGenericAcceptMethod<DoubleVisitable, AllAcceptMethods>
{
    template<typename Visitor> 
    double acceptT(Visitor* visitor) 
    {
        return visitor->visit(*this);
    };

    double m_val = 1.0;
};

struct StringVisitable : public WithGenericAcceptMethod<StringVisitable, AllAcceptMethods>
{
    template<typename Visitor> 
    double acceptT(Visitor* visitor) 
    {
        return visitor->visit(*this);
    };
    double m_val = 0.0;
};